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A Chicago pediatrician who"felt called" to move to Afghanistan to treat children and train physicians was among three Americans killed Thursday at a Kabul hospital by an Afghan guard.

The police guard opened fire on the pediatrician and four others with him at the CURE Hospital's gates, Kabul police said, in violence not unlike the "green-on-blue" attacks in which Afghan security forces fire upon coalition troops.

Two others were killed, and a third person was injured in the hospital attack. A fourth person was unharmed.

Dr. Jerry Umanos practiced medicine in inner-city Chicago before moving to Afghanistan in 2005, according to the U.S. hospital with which he was affiliated, Lawndale Christian Health Center.

Dr. Jerry Umanos had practiced medicine in inner-city Chicago.

"Our family and friends have suffered a great loss and our hearts are aching," his wife, Jan Schuitema, told reporters in Chicago. The doctor and his wife have three children together.

"I know Jerry would also really like everybody to know about his love for the Afghan people, and our love for the Afghan people, and that we don't hold any ill will towards Afghanistan in general, or even the gunman who did this. We don't know what his history is," she said.

Schuitema stressed that her husband's work was an extension of his faith.

"He always had a desire to be the hands and feet of Christ. He was always a light for Christ," she said.

Umanos was connected with Lawndale for more than 25 years, said Dr. Bruce Rowell, the hospital's chief clinical officer.

"He was a loving, caring physician" who served his patients "with the utmost of respect," he said.

Umanos worked at the Kabul hospital as well as at a community health center, the only two training programs for Afghan doctors in the country, according to his biography at Lawndale's website, which noted that he "felt called" to go to Afghanistan nearly a decade ago.

"He loved the country, he loved the people, he loved to teach," a former colleague told CNN. "As much as we love and miss him, all of us have a certain level of respect and contentment knowing Jerry died doing what he loved most."

The senior pediatrician had worked at the hospital in Kabul for seven years.

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Two of those killed Thursday were a father and son, said Suraya Dalil, Afghan minister of public health.

The injured victim has undergone medical treatment and is in stable condition, she added.

The police guard shot himself but survived, police spokesman Hashmatullah Stanekzai said. The motive for the attack was not immediately clear.

U.S. National Security Council spokeswoman Caitlin Hayden condemned the attack.

"Any such attack on civilians at a hospital is despicable and cowardly," she said in a statement.

CURE is a nonprofit organization that operates hospitals and programs in 29 countries, according to its website.

It said patients "experience the life-changing message of God's love for them" while receiving treatment regardless of gender, religion or ability to pay.

The CURE hospital in Kabul has about 100 beds and about 37,000 patients annually.

In Chicago and Afghanistan alike, Umanos knew that patients' circumstances meant that many could not return for follow-up visits, the Lawndale website said.

"In both places, he knows that he must provide the best care possible at each visit, because there is a chance that he may not see the patient again," his bio said.

One colleague who spoke with Umanos just hours before his death said the pediatrician expressed excitement that a training program he developed was finally getting off the ground.

"Jerry and I worked closely for years to develop and implement training programs that provide local Afghan women with basic health education and skills to provide critical health services and best practices in their communities," said Evan A. Russell, a medical student at Johns Hopkins University and co-founder of Empowerment Health, a nonprofit group focusing on Afghan women and children. "Just this morning, he expressed how excited he was that, after years of development with our Afghan partners, we were already on to our second day of training."

More attacks on foreigners

Afghanistan has seen a spate of deadly attacks against foreigners in recent weeks.

On April 4, two Associated Press journalists were shot in Afghanistan's eastern Khost province. Award-winning German photographer Anja Niedringhaus was killed, and Canadian reporter Kathy Gannon was injured.

And last month, five militants set off a deadly car bomb and then stormed a guesthouse used by foreigners in Kabul, the Afghan Interior Ministry said.

The militants held several foreigners hostage, including three Americans, a Malaysian and a person from an unspecified African country.

One girl was killed in the bombing. By the end of the hostage ordeal, one militant was shot and killed, and the other four blew themselves up.

The Taliban claimed responsibility for the March attack. Taliban spokesman Zabiullah Mujahid said militants targeted a location that foreigners used as a church and for converting Afghans to Christianity.